There have been variously proposed stencil printers where print is made by driving, for instance, a thermal head according to image data obtained by reading out an original by, for instance, a scanner to selectively melt and perforate stencil material to make a stencil, winding the stencil around a printing drum, supplying ink inside the printing drum, and transferring the ink to printing papers through the stencil by, for instance, a roller.
In the stencil printers described above, a removable ink container is employed for the reason of easiness in handling or the like. When the ink in an ink container is consumed, the ink container is removed from the stencil printer and a new ink container filled with ink is mounted on the stencil printer.
The ink container comprises, for instance, an ink cylinder provided with an ink discharge port on its leading end and an ink piston slidable along the inner surface of the side wall of the ink cylinder, and the ink is accommodated in the space surrounded by the ink cylinder and the ink piston. In the ink container, as the ink is sucked through the ink discharge port by the ink supply pump, the ink piston is pushed toward the ink discharge port under the atmospheric pressure.
When such an ink container is used, the ink can be accidentally exhausted to deteriorate the workability unless the ink residue is known. Though the ink residue can be visually known, since the ink container is generally installed inside the printer which cannot be viewed from the outside, the printing action must be interrupted to visually know the ink residue, which can also deteriorate the workability. Further, direct measurement of the ink residue in the ink container or the ink consumption adds to the overall size of the printer and at the same time makes it difficult to measure it at high accuracy.
In Japanese Unexamined Patent Publication No. 2002-86678, there has been proposed a method of calculating the ink residue where the difference in light-transmittance between the part where ink adhering to the inner wall of the ink cylinder is scraped off the inner wall of the ink cylinder in response to slide of the ink piston and the part which does not undergo such slide of the ink piston is utilized to calculate the ink residue, and in Japanese Unexamined Patent Publication No. 10(1998)-133529, there has been proposed a method of calculating the ink residue where the position of the ink piston is detected by detecting a metal piece mounted on a side surface of the ink piston by sensors provided along a side surface and the ink residue is calculated on the basis of the position of the ink piston. However, the method disclosed in Japanese Unexamined Patent Publication No. 2002-86678 gives rise to a problem that since adhesion of the ink to the inner wall of the ink cylinder fluctuates, the part where ink adhering to the inner wall of the ink cylinder is scraped off the inner wall of the ink cylinder in response to slide of the ink piston and the part which does not undergo such slide of the ink piston can be mistaken for each other. Further, the method disclosed in Japanese Unexamined Patent Publication No. 10(1998)-133529 gives rise to a problem that since a metal piece is mounted on the ink piston in the ink container, the cost increases and at the same time, it becomes difficult to recycle the ink container after use. Further, since the number of light emitting elements and light receiving elements is limited, accurate calculation of the ink residue at a fine unit is difficult and leads to increase in the cost.
Accordingly, in Japanese Unexamined Patent Publication No. 2001-18507, there has been proposed a method of calculating the ink residue where the working time of the ink supply pump or the number of rotations of the ink supply pump drive motor is measured, the ink discharge from the ink container is calculated on the basis of, for instance, the working time of the ink supply pump and the ink residue is calculated by cumulatively subtracting the ink discharge from the total ink volume accommodated in the ink container.
However, the method disclosed in Japanese Unexamined Patent Publication No. 2001-18507 is disadvantageous in that, for instance, when the ink residue is calculated on the basis of the working time of the ink supply pump, since viscosity of ink changes depending on the kind or the working environmental temperature of the ink and load of the ink on the ink supply motor changes with change in viscosity of the ink, which changes the rotational speed of the drive motor of the ink supply pump, the ink discharge from the ink container for a given working time of the ink supply pump changes depending on the kind or the working environmental temperature of the ink, whereby the ink residue cannot be accurately calculated.
Further, for instance, when ink of low viscosity is accommodated in an ink container, an ink container having a small discharge port can be used since there is a fear that the ink can flow out through the ink discharge port. When the size of the ink discharge port differs depending on the kind of ink, the ink discharge for a unit rotation of the ink supply pump or the drive motor of the ink supply pump differs depending on the kind of ink and accordingly, the ink residue cannot be accurately calculated on the basis of the number of rotation of the drive motor of the ink supply pump.
In view of the foregoing observations and description, the primary object of the present invention is to provide a method of, a system for and an ink container which permit accurate calculation of the ink residue where the ink residue is calculated on the basis of the working time of the ink supply pump or the number of rotations of the ink supply pump drive motor even if the kind of ink and/or the working environmental temperature of the ink changes.